1. Field of the Invention
The present invention relates generally to a voltage-current conversion circuit, and more particularly, a voltage-current conversion circuit outputting a current corresponding to a difference in input voltages applied to a pair of transistors forming a differential circuit.
2. Description of the Related Art
Conventionally, as a voltage-current conversion circuit adapted for a variable low-pass filter, electronic volume and the like, a circuit like that shown in FIG. 1 is known. In FIG. 1, input voltages Vin1, Vin2 are applied to a pair of input terminals 10, 11. Each of the input terminals 10, 11 is connected to the bases of npn transistors Q1, Q2, respectively, the npn transistors Q1, Q2 forming a differential circuit. The collectors of transistors Q1, Q2 are connected to the collectors of transistors Q3, Q4. Transistors Q3, Q4 form a current-mirror circuit, their bases being connected to each other and jointly connected to the collector of Q3 and their respective emitters being connected to a power source Vcc.
An output terminal 14 outputting a current Iout is connected to the collector of the transistor Q4. The emitters of transistors Q1, Q2 are connected both to the two ends of the resistor Rg as well as to a constant current source 12, 13 generating constant currents I, I' (I'.apprxeq.I).
It should be noted that when an input voltage differential dV (=Vin1 minus Vin2) is applied between the input terminals 10, 11 a current Ig (=dV/Rg) corresponding to the voltage dV flows to the resistor Rg, the output current Iout being generated based on the current Ig.
A requirement of the conventional circuit described above is that the input voltages Vin1, Vin2 be lower than the power source voltage Vcc. If this requirement is not met the circuit does not operate properly. As a result, the size of the input voltages Vin1, Vin2 is limited and hence the range of applications of the circuit is limited as well.